How do you get the secret potion in Pokemon Platinum?

After you beat Crasher Wake in Pastoria City, a Galactic Grunt will have set off a bomb Once you meet your friend outside, Crasher Wake will run outside.Run up to the Great Marsh and talk to Wake.While your friend will guard the Marsh, You follow the Grunt until he tires out and battles you.If you beat him, he will run away.Cynthia comes out and after a talk,she gives you the secret potion and a charm to give to her grandmother.